1. Field of the Invention
The general field of the invention is the area of fiber optics. In the present invention fiber optics are used for sensing or detecting vibration, impact or pressure. The sensor system uses a multimode fiber sensor as its sensing element coupled to a light source and a light detector. Intensity variation of optical power in response to perturbation of the sensing fiber via static and dynamic disturbance of the sensing fiber geometry changes the optical energy received by the light detector.
2. Description of the Related Prior Art
Fiber optic sensors can be classified into Extrinsic and Intrinsic fiber optic sensors. (Eric Udd "Fiber Optic Sensors" An introduction for engineers and Scientist 1990).
Extrinsic or hybrid fiber optic sensors are sensors in which light transits into and out of the fiber to reach the sensing region. The fibers serve as a conduit for transmitting the light and the fiber does not come directly into contact with the substrate that is measured. Such inventions are characterized as extrinsic sensors, and are not related to the art of the invention described herein. Examples of extrinsic fiber optic sensors include Laser Doppler velocimetry, Fluorescence and Reflection and Transmission sensors.
Intrinsic or all-fiber optic sensors are sensors in which the environmental effect is converted to a light signal within the fiber. The sensing fibers of Intrinsic fiber optic sensors come directly into contact with the article or substance sensed and any change in the physical environment is translated into a change in the optical transmission through the sensing optical fiber system. The invention described herein falls into this category.
The close analogy to the invention described herein is the multimode fiber microbend intensity sensor. Light loss in the microbend fiber sensing system depends upon reducing the amount of light transmitted through the fiber by distorting the fiber geometry. Distortion of this geometry scatters the light out of the core into the cladding thus decreasing the total light transmitted through the fiber. This is typically achieved by compressing the fiber between two ridged plates, with the ridges of opposing plates arranged in a complementary fashion so as to periodically deform the fiber. Such a microbend intensity sensor is simple and relatively inexpensive, however, it is not a sensitive detector to pressure or vibrations and requires large lengths of sensing fiber coupled to mechanical devices to deform the fiber. The current invention overcomes non-sensitivity of the microbend sensor and can arbitrarily be considered as a "Multimode Fiber Speckle Sensor." Such sensors are describe below:
In "Optical Fiber Communications Systems" by C. P. Sandbank, published by John Wilely & Son of New York, 1980, pp 249-265, describes that in a multimode optical fiber illuminated at a first face by a coherent light, that the light follows different optical paths within the fiber. Each optical path is modulated differently by external disturbances, such as vibrations, temperature andl/or pressure variations. At the output face of the optical fiber, the modes of the multimode optical fiber are combined so as to form an interference pattern with a granular mottled appearance called "speckle." The changes in the speckle produced by external phenomena on the multimode fiber are used to form various "speckle mode sensors" such as:
A Fluid flow measurement sensor, described by J. H. Lyle and C. W. Pitt in "Electronics Letters" of the Mar. 19, 1981. vol 17 no. 6, pp. 244-245. The sensor system described comprised a multimode optical fiber in the fluid stream, illuminated at its input by helium neon laser emitting coherent light. A speckle, formed at the output end of the multimode optical fiber by vibrations produced in the vicinity of the fiber by flow of the fluid adjacent to the multimode fiber, is observed by a photodetector disposed at 15 cm. from this output end, the output signal of the photodetector being displayed on an audiofrequency spectrum analyzer;
Chung-yee Leung, 1-fan Chang and Si-Hsu have described in the "Proceedings of the Fourth International Conference on Optical Fiber Sensors" Tokyo Oct. 7-9, 1986, an intrusion detection device. The device comprises a multimode optical fiber illuminated by a laser whose light is focused on the input end of the multimode optical fiber disposed in a practically closed loop and whose speckle appears on the output face of this fiber. This speckle is detected by several photodetectors of small size disposed in the vicinity of the center of the speckle, the output signals of the photodetectors, being amplified, filtered (by band-pass filters of 10 to 200 Hz), rectified, added and blocked.
In U.S. Pat. No. 4,843,233 issued to Lue Jeunhomme on Jun. 27, 1989 a device for detecting vibrations like those described above (a multimode optical fiber as the sensing element) is proposed. Jeunhomme device is distinguished from the above described sensors by the fact that a monomode optical fiber is provided between the coherent light source and the input face of the multimode optical fiber and at least one optical fiber, having a core diameter smaller than that of the multimode optical fiber, is provided between the output face of this multimode optical fiber, on which the speckle, appears and at least one associated photodetector.
The present invention is similar to the above devices in that it uses a multimode fiber as the sensing element, and distortion in the geometry of the multimode fiber causes changes the speckle pattern detected at the output face of the multimode fiber. What sets the current invention apart from the prior art is the use of a polarization maintaining fiber between the coherent light source and the input face of the multimode sensing optical fiber. Jeunhomme teaches that the presence of the monomode optical fiber between the coherent light source (proximal or source umbilical fiber) and the input face of the multimode optical fiber has the advantage preventing the disturbance that might occur outside his multimode fiber. Jeunhomme's does not teach or disclose that minor disturbances on the proximal (source) umbilical fiber cause significant disturbances on the speckle pattern on the distal end of the multimode fiber. The current invention eliminates the sensitivity problem by using polarization maintaining single mode fiber.